The present invention relates to a method of coating a substrate with a coating material, preferably a metal or a metallic compound, and an apparatus therefor.
Coating a coating material, preferably a substrate with a metal or a metallic compound, is generally conducted for corrosion protection, decoration, reinforcement and the like. The representative examples of methods of coating a substrate of the prior art include electroplating, vacuum evaporation and electrostatic spraying.
Electroplating is a method of depositing a metal by an electrochemical reaction on an electrode dipped in a plating solution. This technique has disadvantages such as the types of coating materials being limited and that a metal coating can only be formed on the order of a few microns. In addition, electroplating is not an economical method because it requires a complicated large-scale system and a large amount of electric power so that production cost is high. When a plating solution containing cyanogen, sodium hydroxide, or ammonia is used, plating efficiency and recovery rate of a coating material are low and waste disposal of the plating solution causes a serious pollution problem. In the case of melt plating, a melted coating material reacts with a substrate to be coated because the coating treatment is conducted at a high temperature.
Vacuum evaporation is a method of vacuum coating by heating a target material placed on a filament or in a crucible by a heating resistor, electron beam or scattered light from a laser, or by ion-sputtering of a target material. Although laser-heating and ion-sputtering can be conducted at a relatively low temperature compared with other vacuum evaporation techniques, they can not eliminate such disadvantages as a crucible causing contamination and coating materials reacting with one another or with a substrate so that an alloy is formed. In addition, since particles vacuum-evaporated or sputtered from a target are active and thus react with residual gas to generate impurities, a coating having high purity can not be obtained. Moreover, coating efficiency and recovery rate of a coating material are low. The coating obtained by this method has low adhesion to a substrate and is brittle. Furthermore, when a substrate having a large area is coated, a coating having a uniform thickness cannot be obtained.
Electrostatic spraying is a method of coating a substrate by spraying a coating solution from a nozzle onto a substrate. This method is simpler than the above two methods. However, electrostatic spraying has the disadvantages that a coating has low adhesion to a substrate and low density. In addition, this method is not economical because it requires special steps such as pre-washing of the substrate surface, pre-treatments for providing the substrate with adherability to a coating, a drying step and the like.
When a substrate in a complicated shape, for example, the inner surface of a hollow cylinder is coated, a uniform coating can not be obtained using any of the above methods.